Concentric camshafts have an inner and an outer camshaft wherein the inner camshaft is used for one set of valves and the outer camshaft is used for another set of valves on an internal combustion engine. Camshaft adjusters or phasers for concentric camshafts are conventional star-type camshaft phase adjusters attached to the outer camshaft. The phaser is used to phase the inner camshaft with respect to the outer camshaft. This means that the phaser can accomplish intake phasing based on the camshaft design and the engine architecture. When attaching the phaser to a concentric camshaft, it is important that the chain load (loads in the direction of the camshaft directly down towards the crankshaft) be transmitted from the phaser, into the more structure-rigid outer camshaft. If the load is supported principally by the inner camshaft, bending will occur on the inner shaft which will cause the inner and outer camshafts to bind to one another and prevent intake versus exhaust valve timing adjustments.
Conventional torsion springs are used to assist the phasing of the inner camshaft to the advanced position from the parked position of retard. Conventional torsion springs used on phasers for concentric twin camshafts are mounted externally to the phaser with one end hooked to the sprocket of the camshaft and the other end is hooked to the camshaft via a tab on the trigger wheel.